Rotary centrifugal pumps are conventionally used in industrial applications to induce flow of fluids via a pressure difference. The maximum pressure head that can be obtained depends on the external diameter of the impeller and the speed of the rotating shaft. Consequently, for high pressure head applications, a large rotary centrifugal pump is required, leading also to high power consumption.
However, it is often not feasible to use a large-sized pump especially where space is a constraint. Furthermore, it is desirable to have as low a power consumption as possible to improve efficiency and save energy.
Due to its valveless nature, ultrasonic pumps have been proposed. As shown in FIG. 1(a) (prior art), an ultrasonic pump 1 comprises chiefly a tube 2 with a plate 3 positioned at a gap G from the tip 4 of the tube 2. Either the tube 2 or the plate 3 is ultrasonically vibrated so as to create a displacement D in the gap G. This generates a pressure P in a region of the fluid 5 immediately between the tip 4 and the plate 3, thereby pushing water into the tube 2 as shown by the block arrow. The pressure P generated is a function of several parameters such as the gap G, internal diameter ID of the tube 2, vibration amplitude D and vibration frequency ƒ used. In an alternative embodiment, the ultrasonic pump comprises the tube 2 with an insertion rod 6 as shown in FIG. 1(b) (prior art).
As an example, an ultrasonic pump from Precision and Intelligence Laboratory of the Tokyo Institute of Technology uses a bending disk transducer to vibrate the plate 3. This achieved a maximum pump pressure of about 2 mH2O (or 20 kPa) with a vibration velocity of 1.0 m/s and a gap size of 10 μm, obtaining a maximum flow rate of 22.5 mL/min with input power of 3.8 W. Another ultrasonic pump from the same source uses a vibrating tube 2 (with or without the insertion rod 6) to achieve a similar maximum pump pressure. Although prototypes have been developed, the maximum pump pressure is still low for many practical applications, such as micro channel cooling.